cir. XXX. SIR WILLIAM HERSCHEL. 271 



matical problem, proved that whatever might be the changes, 

 and they are almost infinite, caused by all the attractions of 

 the different planets on each other, yet in the course of long 

 ages every part of the solar system remains stable. Each 

 planet has its appointed road, along which it travels, through 

 many twists and turnings, but from which it cannot escape, 

 for the grand force of gravitation holds them all in one 

 eternal round about their sun. 



These are some of the problems solved by Lagrange and 

 Laplace. You cannot expect to understand their full signi- 

 ficance, nor must you imagine that these few pages contain 

 more than a very small fraction of the work which these two 

 mathematicians accomplished. Laplace made some beau- 

 tiful calculations explaining the theory of the tides, and you 

 will also often hear him mentioned as the author of the 

 * Nebular Hypothesis,' by which he taught that our earth 

 and all the planets were in the beginning formed by the 

 condensation of gases and fluid matter. All this, which is 

 too difficult to enter upon here, is discussed in his famous 

 work, the 'Mecanique Celeste,' published in 1799. But 

 the main points to be remembered are that Lagrange 

 and Laplace proved the regular order of the movements of 

 the planets, and explained all those anomalies which had 

 seemed to be out of harmony with Newton's theory of gra- 

 vitation. 



Sir William Herschel constructs his own Telescopes, 

 and discovers Uranus, 1781. — William Herschel, who was 

 bom in 1738, was one often children. His father, who was 

 an eminent musician, brought him up to follow his own 

 profession, and when William came over to England with 

 his regiment he started in life as a teacher of music. The 

 first three years in this country were years of hard struggle 



